I'm still amazed when I think that my 6s came with iOS 9 and I've updated to 10, 11, 12, 13 and everything went very smoothly. I'm going to stick on iOS 13.7 as it works very well and I don't need all the bells and whistles of iOS 14 (although I'm sure my 6s can handle it) I've been through many batteries (some my cost, some Apple paid) and it still performs as when I first bought it.
My 6s has been the best iPhone I've ever owned, the 2 GB of ram is a crucial part of it's performance all these years.